A. Field of the Invention
The present invention relates generally to systems for controlling the flow of fluids in wells, and more particularly, to a subsurface system for controlling the flow of fluids in the tubing and the annulus.
B. Description of the Prior Art
Surface controlled subsurface safety systems are used for shutting in wellbores below the wellhead. They all include a valve inserted in the tubing string, which is normally held open, but which closes upon loss of a pressure signal from the surface. The valves may be either included as part of the tubing or insertable into or removable from the tubing by wireline. Valves that are included as a part of the string are commonly referred to as tubing-retrievable subsurface safety valves and include, for example, Johnston-Macco "TF Surface-Controlled Tubing-Retrievable SSSV" as shown at page 4999 of the 1982-83 Composite Catalog or Baker Packers "FVL or FVH Tubing Retrievable Safety Valves", as shown at pages 912-913 of the 1982-83 Composite Catalog. The valves that are insertable into or removable from the tubing by wireline are commonly referred to as wireline retrievable subsurface safety valves and include, for example, Johnston-Macco "WF Surface-Controlled Wireline-Retrievable SSSV" as shown at page 5000 of the 1982-83 Composite Catalog or Baker Packers "BFV Wireline Retrievable Safety Valve" as shown at pages 938-39 of the 1982-83 Composite Catalog. A packer is always set between the tubing and the casing so that, normally, when the safety valve closes, the entire well i.e., tubing and annulus, is sealed.
In some situations, however, it is necessary or desirable to vent the casing or annulus. Venting the annulus is particularly desirable in situations where the formation pressure is very low. When the formation pressure is low, it is necessary to reduce as much as possible the bottom hole pressure so as not to impede the production from the formation. More specifically, the rate of production from the formation is governed in large part by the differential between the formation pressure and bottom hole pressure. As the oil flows out of the formation and into the annulus, gas often comes out of the oil and builds up in the annulus. If the annulus is not vented, then the gas pressure increases the annulus pressure and thereby decreases the differential between formation pressure and bottom hole pressure.
Presently, there exist subsurface safety systems that allow casing or annulus pressure to be vented. Typically, such systems include a combination of a tubing safety valve with an annulus safety valve. More specifically, the presently existing systems include a combination tubing safety valve and vented packer with means for closing the packer vent. When the tubing safety valve is signaled to close, the vent also closes. One example of an existing system is disclosed in U.S. Pat. No. 4,049,052. Additional examples are disclosed in U.S. Pat. No. 3,035,642; U.S. Pat. No. 3,313,350; U.S. Pat. No. 3,252,476; U.S. Pat. No. 3,045,755; U.S. Pat. No. 3,156,300; and U.S. Pat. No. 3,299,955.
There are a number of shortcomings in the presently existing annulus subsurface safety systems. The most serious shortcoming is in that all prior annulus venting systems provide a restricted flow passage that may allow full venting on the case. Additionally, in most systems, the annulus valve is exterior of the tubing and inaccessible from the surface. In such systems, while the tubing subsurface safety valve may be replaced by a wireline retrievable valve, the annulus valve cannot be serviced or replaced without pulling the tubing.
It is therefore an object of the present invention to provide a subsurface safety system that overcomes the shortcomings of the prior art. More specifically, it is an object of the present invention to provide an annulus subsurface safety system that allows full bore venting of the annulus.
It is a further object of the present invention to provide an annulus subsurface safety system wherein both the tubing safety valve and annulus safety valve are independently operable full bore tubing retrievable subsurface safety valves or wireline retrievable subsurface safety valves, both of which may be accessed and serviced from the surface.
It is a further object of the present invention to provide an annulus subsurface safety system which may be serviced in wells with highly deviated angles and heavy, viscous, low gravity crude oil.